Ionospheric correction of interferometric SAR data with application to the cryospheric sciences

Thesis (Ph.D.) University of Alaska Fairbanks, 2018The ionosphere has been identified as an important error source for spaceborne Synthetic Aperture Radar (SAR) data and SAR Interferometry (InSAR), especially for low frequency SAR missions, operating, e.g., at L-band or P-band. Developing effective algorithms for the correction of ionospheric effects is still a developing and active topic of remote sensing research. The focus of this thesis is to develop robust and accurate techniques for ionospheric correction of SAR and InSAR data and evaluate the benefit of these techniques for cryospheric research fields such as glacier ice velocity tracking and permafrost deformation monitoring. As both topics are mostly concerned with high latitude areas where the ionosphere is often active and characterized by turbulence, ionospheric correction is particularly relevant for these applications. After an introduction to the research topic in Chapter 1, Chapter 2 will discuss open issues in ionospheric correction including processing issues related to baseline-induced spectrum shifts. The effect of large baseline on split spectrum InSAR technique has been thoroughly evaluated and effective solutions for compensating this effect are proposed. In addition, a multiple sub-band approach is proposed for increasing the algorithm robustness and accuracy. Selected case studies are shown with the purpose of demonstrating the performance of the developed algorithm. In Chapter 3, the developed ionospheric correction technology is applied to optimize InSAR-based ice velocity measurements over the big ice sheets in Greenland and the Antarctic. Selected case studies are presented to demonstrate and validate the effectiveness of the proposed correction algorithms for ice velocity applications. It is shown that the ionosphere signal can be larger than the actual glacier motion signal in the interior of Greenland and Antarctic, emphasizing the necessity for operational ionospheric correction. The case studies also show that the accuracy of ice velocity estimates was significantly improved once the developed ionospheric correction techniques were integrated into the data processing flow. We demonstrate that the proposed ionosphere correction outperforms the traditionally-used approaches such as the averaging of multi-temporal data and the removal of obviously affected data sets. For instance, it is shown that about one hundred multi-temporal ice velocity estimates would need to be averaged to achieve the estimation accuracy of a single ionosphere-corrected measurement. In Chapter 4, we evaluate the necessity and benefit of ionospheric-correction for L-band InSAR-based permafrost research. In permafrost zones, InSAR-based surface deformation measurements are used together with geophysical models to estimate permafrost parameters such as active layer thickness, soil ice content, and permafrost degradation. Accurate error correction is needed to avoid biases in the estimated parameters and their co-variance properties. Through statistical analyses of a large number of L-band InSAR data sets over Alaska, we show that ionospheric signal distortions, at different levels of magnitude, are present in almost every InSAR dataset acquired in permafrost-affected regions. We analyze the ionospheric correction performance that can be achieved in permafrost zones by statistically analyzing correction results for large number of InSAR data. We also investigate the impact of ionospheric correction on the performance of the two main InSAR approaches that are used in permafrost zones: (1) we show the importance of ionospheric correction for permafrost deformation estimation from discrete InSAR observations; (2) we demonstrate that ionospheric correction leads to significant improvements in the accuracy of time-series InSAR-based permafrost products. Chapter 5 summarizes the work conducted in this dissertation and proposes next steps in this field of research

Applications of SAR Interferometry in Earth and Environmental Science Research

This paper provides a review of the progress in regard to the InSAR remote sensing technique and its applications in earth and environmental sciences, especially in the past decade. Basic principles, factors, limits, InSAR sensors, available software packages for the generation of InSAR interferograms were summarized to support future applications. Emphasis was placed on the applications of InSAR in seismology, volcanology, land subsidence/uplift, landslide, glaciology, hydrology, and forestry sciences. It ends with a discussion of future research directions

Advanced satellite radar interferometry for small-scale surface deformation detection

Synthetic aperture radar interferometry (InSAR) is a technique that enables generation of Digital Elevation Models (DEMs) and detection of surface motion at the centimetre level using radar signals transmitted from a satellite or an aeroplane. Deformation observations can be performed due to the fact that surface motion, caused by natural and human activities, generates a local phase shift in the resultant interferogram. The magnitude of surface deformation can be estimated directly as a fraction of the wavelength of the transmitted signal. Moreover, differential InSAR (DInSAR) eliminates the phase signal caused by relief to yield a differential interferogram in which the signature of surface deformation can be seen. Although InSAR applications are well established, the improvement of the interferometry technique and the quality of its products is highly desirable to further enhance its capabilities. The application of InSAR encounters problems due to noise in the interferometric phase measurement, caused by a number of decorrelation factors. In addition, the interferogram contains biases owing to satellite orbit errors and atmospheric heterogeneity These factors dramatically reduce the stlectiveness of radar interferometry in many applications, and, in particular, compromise detection and analysis of small-scale spatial deformations. The research presented in this thesis aim to apply radar interferometry processing to detect small-scale surface deformations, improve the quality of the interferometry products, determine the minimum and maximum detectable deformation gradient and enhance the analysis of the interferometric phase image. The quality of DEM and displacement maps can be improved by various methods at different processing levels. One of the methods is filtering of the interferometric phase.However, while filtering reduces noise in the interferogram, it does not necessarily enhance or recover the signal. Furthermore, the impact of the filter can significantly change the structure of the interferogram. A new adaptive radar interferogram filter has been developed and is presented herein. The filter is based on a modification to the Goldstein radar interferogram filter making the filter parameter dependent on coherence so that incoherent areas are filtered more than coherent areas. This modification minimises the loss of signal while still reducing the level of noise. A methodology leading to the creation of a functional model for determining minimum and maximum detectable deformation gradient, in terms of the coherence value, has been developed. The sets of representative deformation models have been simulated and the associated phase from these models has been introduced to real SAR data acquired by ERS-1/2 satellites. A number of cases of surface motion with varying magnitudes and spatial extent have been simulated. In each case, the resultant surface deformation has been compared with the 'true' surface deformation as defined by the deformation model. Based on those observations, the functional model has been developed. Finally, the extended analysis of the interferometric phase image using a wavelet approach is presented. The ability of a continuous wavelet transform to reveal the content of the wrapped phase interferogram, such as (i) discontinuities, (ii) extent of the deformation signal, and (iii) the magnitude of the deformation signal is examined. The results presented represent a preliminary study revealing the wavelet method as a promising technique for interferometric phase image analysis

Fusion of remotely sensed displacement measurements: current status and challenges

International audienc

Studying Changes in the Cryosphere Using Radar Interferometry: Permafrost Surface Subsidence and Glacial Unloading Deformation

In past decades, cryospheric components such as glaciers, ice sheets, sea ice, and frozen ground have been undergoing significant and rapid changes, associated with changes in the global climate system. In this research, I present two case studies on how geodetic tools, especially Radar Interferometry (InSAR), can be used to monitor and to advance our understanding of the changing cryosphere. First, I measure seasonal and long-term surface subsidence on the North Slope of Alaska near Prudhoe Bay using InSAR. I detect a long-term surface subsidence of 1 to 4 cm per decade, which is probably caused by melting of ground ice within the permafrost, as ground temperatures have increased by 2 to 3°C in northern Alaska since the early 1980s. I also find a seasonal subsidence of 1 to 4 cm during summer thaw seasons, which is caused by the volume decrease of the top soil layer (the active layer) undergoing annual thawing-freezing cycles. A retrieval algorithm is developed to estimate the active layer thickness (ALT) using the InSAR-measured seasonal subsidence. The estimated ALT values match in situ measurements at Circumpolar Active Layer Monitoring sites within the uncertainties. I estimate an ALT of 30 to 50 cm over moist tundra areas, and a larger thickness of 50 to 80 cm over wet tundra areas. Second, I use InSAR and Global Positioning System data to measure the crustal elastic uplift near Jakobshavn Glacier in west-central Greenland caused by its rapid ice loss since 1997. These geodetic measurements place valuable constraints on the ice mass balance estimation based on altimetry measurements from NASA\u27s Airborne Topographic Mapper (ATM), as I find good agreement between the observed crustal rebound rates and the predicted rates using the ATM measurements. I also directly invert for the spatial pattern of ice thinning from the InSAR-measured crustal deformation. Overall, this research suggests that InSAR-measured surface deformation complements traditional in situ monitoring of the active layer and the permafrost, extends ALT estimates over large areas at high spatial resolution, provides new insights into the dynamics of permafrost systems and changes in permafrost conditions, and helps to study the ice mass loss of a rapidly thinning glacier and its surrounding region and to better understand a glacier\u27s rapid response to a warming climate

Estimating Strain and Rotation tensors of glacier flow from wrapped SAR interferograms

This letter aims to discuss a general framework that allows the direct interpretation of the wrapped DInSAR phase in terms of surface strain S and rotation R components. The methodology is demonstrated showing the estimation of strain and rotation components of a glacier flow using three TerraSAR-X interferometric geometries (ascending right-looking, descending right-looking and descending left-looking. Finally since the left looking geometry can be difficult to obtain on a regular basis, the surface parallel flow assumption is extended to the phase gradients inversion in order to reduce the amount of necessary geometries from three to two

Measuring velocities of a surge type glacier with SAR interferometry using ALOS-2 data

In recent years, in-situ measurements on Kongsvegen, a surge-type glacier located in the Kongsfjorden area, have showed an acceleration in the flow speeds of the glacier. This could indicate the onset of a surging event, which presents the opportunity to study the dynamics of a glacier surge using remote sensing techniques with in-situ data for reference. Synthetic aperture radar (SAR) is well suited for this, as it does not rely on the sun for illumination and is not obstructed by clouds. In addition, SAR can be used to measure displacement with high accuracy and resolution through the use of interferometric SAR (InSAR). This study investigates the acceleration of Kongsvegen using InSAR, MAI and offset tracking. Velocity measurements from the combination DInSAR - MAI are then compared to in-situ data as well as the offset tracking measurements. For image pairs where InSAR measurements are not possible due to phase decorrelation, offset tracking is attempted as a back-up. Data from 2015, 2018 and 2019 was available, and the evolution of flow speeds over time could therefore be evaluated. The image pairs from 2018-2019 were acquired with 14 days separation in time, while the 2015 image pairs were acquired with 28 and 42 days separation. Due to the longer separation in time, the 2015 image pairs decorrelated in time. In addition, a pair acquired in the summer of 2018 decorrelated as a result of surface melting on the glaciers. Therefore only 3 of the total 8 pairs available were suited for interferometric analysis. For the image pairs from 2018-2019, the InSAR measurements were in good agreement with the in-situ data, as they also indicated an acceleration of the flow speeds on Kongsvegen. The offset tracking results on these pairs overestimated the velocity magnitudes, but also showed an increase in time. Similar to the InSAR estimates, the offset tracking failed to produce reasonable results on the 2015 image pairs, likely because of the large temporal baseline and lack of surface features on Kongsvegen. Overall, InSAR could be used to measure flow speeds on Kongsvegen successfully, but more data with a short temporal baseline is needed for an in-depth analysis

Multi-Temporal investigation of the Boulder Clay Glacier and Northern Foothills (Victoria Land, Antarctica) by integrated surveying techniques

The paper aims to detect the main changes that occurred in the area surrounding the Mario Zucchelli Station (MZS) through analysis of multi-temporal remote sensing integrated by geophysical measurements. Specific attention was directed at realizing an integrated geomorphological study of the Boulder Clay Glacier, a partially debris-covered glacier belonging to the Northern Foothills (Victoria Land, Antarctica). This area was recently chosen as the location for the construction of a new semi-permanent gravel runway for MZS logistical airfreight operations. Photogrammetric analysis was performed by comparing three historical aerial photogrammetric surveys (carried out in 1956, 1985, and 1993) and Very High Resolution (VHR) GeoEye-1 satellite stereo-image coverage acquired in 2012. The comparison of geo-referenced orthophoto-mosaics allowed the main changes occurring in some particular areas along the coast nearby MZS to be established. Concerning the study of the Boulder Clay Glacier, it has to be considered that glaciers and moraines are not steady-state systems by definition. Several remote sensing and geophysical investigations were carried out with the main aim of determining the general assessment of this glacier: Ground Penetrating Radar (GPR); Geodetic Global Positioning System (GPS) network; multi-temporal satellite Synthetic Aperture Radar (SAR) interferometry. The analysis of Boulder Clay Glacier moraine pointed out a deformation of less than 74 mm y-1 in a time span of 56 years, value that agrees with velocity and deformation data observed by GPS and InSAR methods. The presence of unexpected brine ponds at the ice/bedrock interface and the deformation pattern observed in the central part of the moraine has to be monitored and studied, especially under the long-term maintenance of the future runway

Temporal fluctuations in the motion of Arctic ice masses from satellite radar interferometry

This thesis considers the use of Interferometric Synthetic Aperture Radar (InSAR) for surveying temporal fluctuations in the velocity of glaciers in the Arctic region. The aim of this thesis is to gain a broader understanding of the manner in which the flow of both land- and marine-terminating glaciers varies over time, and to asses the ability of InSAR to resolve flow changes over timescales which provide useful information about the physical processes that control them. InSAR makes use of the electromagnetic phase difference between successive SAR images to produce interference patterns (interferograms) which contain information on the topography and motion of the Earth's surface in the direction of the radar line-of-sight. We apply established InSAR techniques (Goldstein et al., 1993) to (i) the 925 km2 LangjÖkull Ice Cap (LIC) in Iceland, which terminates on land (ii) the 8 500 km2 Flade Isblink Icecap (FIIC) in Northeast Greenland which has both land- and marine-terminating glaciers and (iii) to a 7 000 km2 land-terminating sector of the Western Greenland Ice Sheet (GrIS). It is found that these three regions exhibit velocity variations over contrasting timescales. At the LIC, we use an existing ice surface elevation model and dual-look SAR data acquired by the European Remote Sensing (ERS) satellite to estimate ice velocity (Joughin et al., 1998) during late-February in 1994. A comparison with direct velocity measurements determined by global positioning system (GPS) sensors during the summer of 2001 shows agreement (r2 = 0.86), suggesting that the LIC exhibits moderate seasonal and inter-annual variations in ice flow. At the FIIC, we difference pairs of interferograms (Kwok and Fahnestock, 1996) formed using ERS SAR data acquired between 15th August 1995 and 3rd February 1996 to estimate ice velocity on four separate days. We observe that the flow of 5 of the 8 outlet glaciers varies in latesummer compared with winter, although flow speeds vary by up to 20 % over a 10 day period in August 1995. At the GrIS, we use InSAR (Joughin et al., 1996) and ERS SAR data to reveal a detailed pattern of seasonal velocity variations, with ice speeds in latesummer up to three times greater than wintertime rates. We show that the degree of seasonal speedup is spatially variable and correlated with modeled runoff, suggesting that seasonal velocity changes are controlled by the routing of water melted at the ice sheet surface. The overall conclusion of this work is that the technique of InSAR can provide useful information on fluctuations in ice speed across a range of timescales. Although some ice masses exhibit little or no temporal flow variability, others show marked inter-annual, seasonal and even daily variations in speed. We observe variations in seasonality in ice flow over distances of ~ 10 km and over time periods of ~10 days during late-summer. With the aid of ancillary meteorological data, we are able to establish that rates of flow in western Greenland are strongly moderated by the degree of surface melting, which varies seasonally and secularly. Although the sampling of our data is insufficiently frequent and spans too brief a period for us to derive a general relationship between climate and seasonality of flow, we show that production of meltwater at the ice surface and its delivery to the ice bed play an important role in the modulation of horizontal flow speeds. We suggest that a similarly detailed investigation of other ice masses is required to reduce the uncertainty in predictions of the future Arctic land-ice contribution to sea level in a warming world

Classification of rock glaciers in southern Colorado based on ice content using radar interferometry and thermal remote sensing

Remote sensing provides a means of assessing potential water resources stored in alpine ground ice; this study focuses on rock glaciers, in particular. A rock glacier is a landform composed of block of loose debris (talus) cemented with ice. There are many ways of classifying rock glaciers; categorizing them based on activity provides context on their movement and ice content. Active rock glaciers are able to flow due to their ice content, while inactive or relict rock glaciers are unable to flow due to lack of sufficient ice. This study uses satellite based radar interferometry to identify and quantify movement of 87 rock glaciers on seven peaks in Southern Colorado. Once the active flowing rock glaciers and inactive nonflowing rock glaciers had been identified, the thermal properties of each group were studied to determine if it was possible to classify rock glaciers based on activity using satellite based thermal imaging. This was accomplished by comparing the amplitude of variation in land surface temperature derived from Landsat 7 and Landsat 8 to daily NOAA weather observations over different periods. Active rock glaciers demonstrated less variation in temperature annually than inactive rock glaciers, likely due to the ice modulating surface temperatures from below. Because rock glacier ice content affects land surface temperature over a period of 1 year, the depth to the ice was estimated using a skin depth calculation to be between 4.8m and 6.9m. Active and inactive rock glaciers appear to have different thermal characteristics that can be identified in satellite based thermal infrared imagery. Identifying the difference between active and inactive rock glaciers could be important in identifying potential water resources in remote alpine ecosystems, and on Mars, as well as provide insight to the climatic history of the region